The present invention relates to a magneto ignition system and more particularly to an improved magneto and associated electronic circuitry which provides ignition advance and retard spark timing, engine speed limiting, and reverse engine rotation protection.
The ignition system as well as the magneto of this invention is particularly useful with small engines of the type commonly used in chain saws, snow blowers, portable power generators, lawn mowers and the like. While the preferred embodiment of the invention is described for use with a two cylinder engine, this is not a limitation of the invention in that the invention may be extended to a multi-cylinder engines by incorporation of a distributor of a known type or use a plurality of magnetos with each of the cylinders.
There are many existing ways of accomplishing ignition timing for both large and small internal combustion engines. Conventional ignition systems comprise a pair of hard metal breaker points which are activated by a rotor which is rotated in synchronism with the engine cycle. This system is inefficient in that the breaker points have limited lifetime and need to be replaced at regular intervals because of their rapid deterioration. Such systems have been replaced by magneto, capacitive discharge ignition systems comprising a rotor which produces a varying magnetic field in a closely spaced pickup coil.
Some prior magneto capacitive discharge ignition systems which change the ignition timing do so by some mechnical means or they effect a change in timing in response to the amplitude of an electrical signal. This generally results in a system that requires calibration. Typically, the change in ignition timing is gradual over the speed of the engine. This application is intended for application requiring a step change in ignition timing at a precise speed. Systems requiring calibration upon manufacture, result in some labor costs and the possibility of calibration change when in use. This invention removes the need for calibration upon manufacture and any susceptibility to change in calibration when installed on an engine.
The present invention accomplishes the functions of ignition advance and retard spark timing, engine speed limiting and reverse engine rotation protection utilizing a uniquely constructed magneto that generates a walking or variable phase waveform in cooperation with a novel circuit.
Accordingly, the present invention comprises an improvement in such magneto capacitive discharge ignition systems and comprises:
A capacitive discharge magneto ignition system comprising at least one ignition coil comprising a primary winding and a secondary winding; and a magneto. The magneto includes a stator and a rotor that is rotated synchronously with the engine for inducing voltages into a plurality of windings supported by the stator. The magneto further includes means for generating a first induced voltage; and means for generating a second induced voltage having a phase which varies relative to the first induced voltage in proportion to the speed of the engine. More particularly the magneto may include: first means for generating a first induced voltage signal characterized by a first portion or first trigger pulse of a first polarity followed by a second portion of opposite polarity and a third portion or second trigger pulse of the first polarity and second means for generating a second induced voltage signal characterized by a first portion or first trigger pulse of the first polarity followed by a second portion of the opposite and a third portion or second trigger pulse having a phase which varies relative to the second trigger pulse to the first induced voltage signal in proportion to the speed of the engine, and third means for generating a third induced voltage signal of polarity opposite to the polarity of said first induced voltage signal. The system further includes: a circuit connecting the magneto to the ignition coil for generating a spark in response to the induced voltages; and includes a charge capacitor charged by the third means during periods when the third induced voltage signal is of the first polarity and timing means responsive to the first and second trigger pulses from the first means and the second means for advancing the timing of discharging of the capacitor through the ignition coil to provide an ignition spark.
The system further includes means responsive to the trigger pulses generated by the first means, referred to above, for inhibiting the third means for charging the charge capacitor when the engine is rotating at or near a maximum desirable speed. This action thereby effectively limiting the engine speed to such maximum desirable level.
It is the object of the present invention to automatically advance the ignition spark at or above nominal engine speed. Such spark advance timing is needed since it may be desirable to increase the available power from the engine. A further object of the invention is to provide an improved magneto comprising a pickup and a rotor which together produce a plurality of trigger pulses which may be used to automatically advance the ignition timing as well as to provide engine overspeed and reverse engine rotation protection in cooperation with the control circuitry disclosed below. A further object of the present invention is to provide an ignition system which does not require calibration during assembly. An additional object of the present invention is to automatically control the ignition retard timing below a certain nominal engine speed to allow for easy starting of the engine.
Many other objects, advantages and purposes of the invention will be clear from the detailed description of the drawings.